Northern Hemisphere Glaciation during the Globally Warm Early Late Pliocene

The early Late Pliocene (3.6 to ~3.0 million years ago) is the last extended interval in Earth's history when atmospheric CO2 concentrations were comparable to today's and global climate was warmer. Yet a severe global glaciation during marine isotope stage (MIS) M2 interrupted this phase of global warmth ~3.30 million years ago, and is seen as a premature attempt of the climate system to establish an ice-age world. Here we propose a conceptual model for the glaciation and deglaciation of MIS M2 based on geochemical and palynological records from five marine sediment cores along a Caribbean to eastern North Atlantic transect. Our records show that increased Pacific-to-Atlantic flow via the Central American Seaway weakened the North Atlantic Current and attendant northward heat transport prior to MIS M2. The consequent cooling of the northern high latitude oceans permitted expansion of the continental ice sheets during MIS M2, despite near-modern atmospheric CO2 concentrations. Sea level drop during this glaciation halted the inflow of Pacific water to the Atlantic via the Central American Seaway, allowing the build-up of a Caribbean Warm Pool. Once this warm pool was large enough, the Gulf Stream–North Atlantic Current system was reinvigorated, leading to significant northward heat transport that terminated the glaciation. Before and after MIS M2, heat transport via the North Atlantic Current was crucial in maintaining warm climates comparable to those predicted for the end of this century

The distribution of long-chain n-alkan-2-ones in peat can be used to infer past changes in pH

Long-chain (C21-C33) n-alkan-2-ones are biomarkers ubiquitous in peat deposits. However, their paleoenvironmental significance lacks constraints. Here we evaluate the influence pH exerts on the occurrence of long-chain n-alkan-2-ones in Chinese peats. A comparison of the distribution in a collection (n= 65) of modern peat samples with different pH (pH values 4.4-8.6) from China demonstrates that their distribution is significantly different in acid compared to alkaline peat. This difference can be explained by the pH control on the conversion of n-alkan-2-one precursor compounds (n-alkanes and fatty acids). Transfer functions between pH and n-alkan-2-one ratios were established using linear and logarithmic regression models. We then applied these proxies to reconstruct variations of paleo-pH in the Dajiuhu peat sequence to identify the history of peatland acidification over the last 13 kyr. We find significant changes in paleo-pH during the deglaciation/early Holocene and relate these to times of dry climate in the region. The drought-induced peat acidification is supported by observations from modern drying events in the peatland. We propose that long-chain n-alkan-2-ones in peats have potential to trace paleo-pH changes across the deglaciation and Holocene, although further research from different peatlands and time periods is still needed

The Marine Isotope Stage 19 in the mid-latitude North Atlantic Ocean: astronomical signature and intra-interglacial variability

n/

Distribution and carbon isotopic composition of diploptene from epiphytic bryophytes in Wuhan, central China

Diploptene is a ubiquitous hopanoid in the geosphere, synthesized by all hopanoid-containing bacteria. Variations in the concentration and stable carbon isotopic composition (δ13C) of diploptene in ancient peats and lignite can be used to reconstruct certain aspects of the wetland methane cycle in the past. However, the sources and mechanisms that control diploptene δ13C values in wetlands are not fully constrained. To address this, here we determined the distribution and δ13C values of diploptene, as well as n-alkanes, obtained from five genera of epiphytic bryophytes (non-vascular plants such as mosses) that occupy-three different habitats: soil, rock, and tree bark. Our data show that the concentrations of diploptene are highly variable with two order of magnitude differences between the various species. Mosses collected from the soil habitat had higher concentrations compared to those from rock and tree habitats. This suggests that the input from some habitats might dominate the sedimentary signal. The δ13C values of diploptene (δ13Cdip) also vary between species with values ranging between –39.2‰ and –31.2‰. Generally, the δ13C values of diploptene and long chain n-alkanes (i.e., C29 and C31) are similar (±2‰) in most of the bryophyte species. This may suggest that diploptene is produced by heterotrophic bacteria that live in symbiosis with the mosses. However, for some bryophytes the δ13Cdip values are much more 13C depleted (>–2‰) compared to long chain n-alkanes, implying that for some mosses bacterial methanotrophs or methylotrophs may contribute to the diploptene pool. Our findings expand our understanding of the biological sources of diploptene in terrestrial epiphytic bryophytes, which will allow for a more detailed interpretation of the long chain n-alkanes and diploptene (δ13C values) in past environmental and paleoclimatic reconstructions

Warming drove the Expansion of Marine Anoxia in the Equatorial Atlantic during the Cenomanian Leading up to Oceanic Anoxic Event 2

Oceanic Anoxic Event (OAE) 2 (~93.5 millions of years ago) is characterized by widespread marine anoxia and elevated burial rates of organic matter. However, the factors that led to this widespread marine deoxygenation and the possible link with climatic change remain debated. Here, we report long-term biomarker records of water column anoxia, water column and photic zone euxinia (PZE), and sea surface temperature (SST) from Demerara Rise in the equatorial Atlantic that span 3.8 million years of the late Cenomanian to Turonian, including OAE 2. We find that total organic carbon (TOC) contents are high but variable (0.41&ndash;17 wt. %) across the Cenomanian and increase with time. This long-term TOC increase coincides with a TEX86-derived SST increase from ~ 35 to 40 &deg;C as well as the episodic occurrence of 28,30-dinorhopane (DNH) and lycopane, indicating warming and expansion of the oxygen minimum zone (OMZ) predating OAE 2. Water column euxinia persisted through much of the late Cenomanian, as indicated by the presence of C35 hopanoid thiophene, but only reached the photic zone during OAE 2, as indicated by the presence of isorenieratane. Using these biomarker records, we suggest that water column anoxia and euxinia in the equatorial Atlantic preceded OAE 2 and this deoxygenation was driven by global warming.</p